Transition Disk Chemistry in the Eye of ALMA

Transcription

1 COURTESY NASA/JPL-CALTECH Spectroscopy2011 January 16, 2011 Transition Disk Chemistry in the Eye of ALMA Ilse Cleeves Univ. of Michigan ADVISOR: Edwin Bergin

2 Outline I. Transition Disks: Introduction II. Model Framework III. First Results Using CB 26: Test Case Disk IV. Chemical Results: Implications V. Observables - The Power of ALMA COURTESY NASA/JPL-CALTECH

3 I. Circumstellar Disks in Transition H-Band Circumstellar disks observed sites of planet formation. Once a protoplanet is massive enough it can dynamically alter the disk gaps, holes, etc. The initial stages of planet formation involves grain growth and reduction of opacity Disks with gaps are called Transition Disks. Disk chemistry will respond to the change in physical conditions set initial chemical conditions at the point of planet formation. Prediction: The clearing of the inner-disk directly reveals the dense (and normally cold) gas rich midplane to the star. Thalmann et al. 2010: imaging the wall in LkCa 15; consistent with SED disk models the result of forming protoplanets? Is this an observable effect? Must first be able to resolve this... ALMA has the power to resolve the gap can directly probe evolving region.

4 Courtesy Kevin Jardine Top-down view of the Galactic Plane in the Gould Belt region. Molecular Clouds HII Regions Star Clusters Nearest SFR: Resolving the Gap

5 Courtesy Kevin Jardine ORION TAURUS- AURIGA PERSEUS 20 AU 50 AU 100 AU CHAMELEON OPHIUCHUS Maximal resolving power today ~500m Baseline Nearest SFR: CO 2-1: 230 GHz Resolving the Gap

6 Courtesy Kevin Jardine ORION TAURUS- AURIGA PERSEUS 1 AU 5 AU CHAMELEON OPHIUCHUS Full ALMA: 12km Baseline Nearest SFR: CO 2-1: 230 GHz Resolving the Gap

7 III. Chemical Model Recipe hν hot surface cold midplane DISK PHYSICS Size, Mass Disk Structure: e.g. D Alessio disks (along with many other SED motivated models). DISK CHEMISTRY Dust (e.g. Weingartner & Draine), composition and settling. Shape of the UV field, continuum and line (Bethell), stellar and ISRF. Relevant chemical network (Fogel et al. 2011): ~6000 Reactions, ~600 Species. END PRODUCTS: Observables? Can calculate resulting line emission: LIME (Brinch et al. 2010).

8 log(flux erg/cm 2 /s/a) III. UV-Field: Continuum UV Monte Carlo Calculation UV drives the chemistry through processes such as photodissociation and photodesorption. z (AU) Dependent on dust composition (opacity) and dust settling many young disks highly settled. Transition disks structurally evolved require full M.C. radiative transfer treatment. Ly α R (AU) τuv >> 1 Include a separate treatment for Ly α photons which behave differently (Bethell & Bergin 2011; Fogel et al. 2011). TW Hydra Wavelength (A)

9 IV. CB 26: An Overview Disk at the edge of a Bok Globule CB 26, 10 North of Taurus/Auriga dark cloud, D 140 pc. Circumstellar disk - edge on. R 200AU 2 Rgap 45AU 2 Mdisk 0.1 Msol 1 L* > 0.5 Lsol 3 M* 0.5 ± 0.1 Msol 1 Mdust 3e-3 Msol 2 Specific model but typical T Tauri parameters used for model calculation can generalize results. Sauter et al Launhardt & Sargent 2001; 2 Sauter et al. 2009; 3 Stecklum et al. 2004

10 IV. CB 26 Model Sauter et al used spatially resolved maps in the NIR and mm along with the object s SED to create a consistent physical model for the system. Midplane illuminated by star α = 2.2 ± 0.1 β = 1.4 ± 0.1 { ~ Pluto s orbit.

11 IV. CB 26 Model Sauter et al used spatially resolved maps in the NIR and mm along with the object s SED to create a consistent physical model for the system. Stellar heating Dense midplane normally cold (~15K) at large distances from star. Sublimation Temperature NH 3 CS HCN, H 2 CO N Pressure (bar) 10-6 CO Wall heated to T = 30-50K, species that would typically be frozen out at the midplane can be sufficiently heated desorb from grains

12 CO HCO+ HCN H2CO N2H+ H2O Chemical model results: gas phase molecules at the wall!

13 LIME Results: C 18 O: J=2-1 Pre-ALMA Res - 140pc with a 500m baseline i = π

14 LIME Results: C 18 O: J=2-1 ALMA-Era Res - 140pc with a 12km baseline i = π

15 C 18 O: J = 6-5 a=0.01 v = 658.6GHz D = 140pc Ideal (τdust << 1) observation. Actual observations will be dependent on inclination angle. Dust optically thick near the midplane. Edge on disk obs. at 0.5 mm with column R = 200AU, density ~ 1e14 g/cm -3 : τdust, 0.5mm > 1 for z < 15AU. However, only the case for nearly edge on systems. No significant dust obstruction for midplane emission for i ~ This corresponds to ~93% of disks observed on the sky.

16 Summary Observe gaps in dust... ALMA provides us with the ability to significantly increase the resolved sample size of planetforming disks. Will allow us to gain a full picture of the evolutionary process of disk dispersal and planet formation. High sensitivity allows us to go both deeper, and to use new tracers not previously employed due to sensitivity limitations. The future: warm molecular rings! Wall will distinctly light-up at high J states - can selectively probe the transition region. The interface between the inner gap and outer disk (the wall) in transition disks should be a chemically active and interesting region that sets the initial conditions for protoplanet chemistry.